The present invention relates to an apparatus for the preparation of a working, processing or machining liquid for a spark erosion or electroerosion machine.
Electroerosion machines typically have a working container filled with a machining or processing liquid into which is immersed a workpiece to be machined by electroerosion. In the case of wire cutting or wire eroding machines, the machining liquid consists of deionized water, whereas it consists of hydrocarbons for countersinking machines. As a result of the erosive removal of particles from the workpiece, the cleanness of the machining liquid is reduced, so that its electrical conductivity changes.
In order to maintain a desired quality of the electroerosion operation, it is necessary to clean or purify the machining liquid, so as to maintain its electrical characteristics and cleanness within predetermined ranges.
The prior art has generally made use of conventional filters for the preparation of the machining liquid; those filters have, however, a comparatively large pore diameter. In the case of the large pore sizes of such conventional filters, the filtering action is not sufficient to maintain the machining liquid of an electroerosion machine within a desired quality range for a period of several days. However, the filter walls become clogged on reducing the pore diameter of the conventional filters for increasing the filtration quality.
It is generally known in the filter technology field to use membrane filters, which have extremely small pore-cross-sections compared with the pore sizes of conventional filters of other types. However, membrane filters have been considered unsuitable for the coarse filtering of the concentrate in electroerosion technology because of the fear of a rapid clogging of the membrane filter due to the high solids proportion in the concentrate.
Therefore membrane filters have only been used for extremely specialized purposes in the preparation of the machining liquid for electroerosion machines but never for cleaning or preparing the concentrate. For example, JP-OS 62-24917 discloses an apparatus for the preparation of the machining liquid of an electroerosion machine, in which use is made of a membrane filter. The known apparatus comprises a container for the machining liquid, which is subdivided into two areas by a cellophane membrane. The first area is connected by means of a pump, downstream of which a convention filter is connected to the working zone of the electroerosion machine which, by means of a further pump, can again be discharged into said first area of the container. The second container area is connected by still another pump to an ion exchanger. The ion concentration in the second container area is well below the ion concentration in the first container area, so that it is possible to operate the ion exchanger with a favorable, low ion density for its operating behavior. In this known apparatus for the preparation of the machining liquid, a conventional filter is consequently used for filtering the concentrate, whereas a member filter is used only for the adjustable reduction of the ion density for a circulation of an ion exchanger.
It has been known in the field of filter technology using membrane filters to carry out a periodic backflushing of the membranes, so that a dirt cake which has accumulated during operation of the membrane can be washed away. In known membrane filtration plants, a time control has been used for this purpose to control a brief backflushing process at the end of a predetermined filtration time. However, in such a filtration plant, the backflushing time intervals can only be set in an economic manner, if the dirt cake builds up on the membrane wall at a substantially invariable speed on the basis of a forecastable, constant filter cake production.